Abstract: A coupling condition of cyclic major and intermediate principal stresses was applied to simulate the coupling of vertical cyclic normal stress with the horizontal cyclic normal stress parallel to the road direction under traffic load in three-dimensional stress space. A stress path with coupling of cyclic major and intermediate principal stresses was then proposed, and the coefficient of cyclic intermediate principal stress bcyc was defined as the characterization parameter of cyclic stress path. A series of undrained cyclic true triaxial tests under coupling of cyclic major and intermediate principal stresses was performed in normally consolidated saturated soft clay by GDS cyclic true triaxial apparatus. The effects of coefficient of cyclic intermediate principal stress and cyclic stress ratios on the long-term shakedown characteristics of normally consolidated soft clay under undrained conditions were analyzed. The experimental results show a significant increase of resilient modulus induced by increasing the coefficient of cyclic intermediate principal stress, which restrains the development of cumulative major principal strain. The cumulative major principal strain has a linear relationship with the coefficient of cyclic intermediate principal stress in the three-dimensional stress state. On the basis of the shakedown theory, the allowable cyclic stress ratios under coupling of cyclic major and intermediate principal stresses were determined, and it was also found that the allowable cyclic stress ratios increased with increasing the coefficient of cyclic intermediate principal stress.

Key words: true triaxial tests, saturated soft clay, cumulative strain, resilient modulus, allowable cyclic stress ratios